The invention generally relates to preparing a bonding surface of a semiconductor layer of a wafer. In particular, the invention concerns the preparation of the bonding surfaces of two wafers of semiconductor materials to create structures to be used in microelectronics, optics and optoelectronics.
To ensure quality contact prior to bonding two wafers, the bonding surfaces must be cleaned. Traditional cleaning techniques include immersing the wafers in baths of successive cleaning solutions.
It is known to clean semiconductor-material wafers by using a so called RCA treatment. Such treatment includes an initial bath of an SC1 solution (Standard Clean 1), containing ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) and deionized water. A second bath is then used, called an SC2 solution (Standard Clean 2), containing hydrochloric acid (HCl), hydrogen peroxide (H2O2) and deionized water. The initial bath principally removes isolated particles from the surface of the wafer and encrusted particles near the surface, and prevents them from resettling. The SC2 solution principally removes metallic contamination that may have settled on the surface of the wafer, which may form chlorides.
However, RCA treatments sometimes are insufficient, as such treatments remove little or none of the organic contaminants. Further, the organic contaminants can inhibit the efficient removal of matter by the RCA treatment. The contaminants can also adversely affect the quality of the bonding of the wafers if on the bonding interface. This occurs when one of the two wafers has been subjected to an implantation of atomic species near a bonding surface and then is followed by detachment of the implanted zone during a SMART-CUT® process. Hydrocarbons can, in the presence of particles, lead to the development of superficial blisters on the wafer. The blisters are typically created after detachment and may occur on non-transferred zones between the zone near where the species were implanted and the surface of the wafer. The blisters increase and/or are amplified during a heat treatment, such as a heat treatment performed during bonding to strengthen the bond. The hydrocarbons, or organic contaminants, may cause bonding problems.
Wafers to be bonded are usually immersed in a bath of CARO, which is a solution that contains H2SO4 and H2O2, at a temperature in excess of 100° C. to remove organic contaminants. This solution is expensive and poses safety and environmental problems. Furthermore, the CARO solution is difficult to apply in some cases, for example, when processing twelve-inch wafers. Moreover, cleaning processes using the CARO solution require intermediate stages between the CARO solution bath and the RCA treatment. Such stages include subjecting the wafer to at least two rinsing baths to remove any trace of the CARO solution present on the surface, prior to immersing the wafer in the SC1 solution.
Thus, a known cleaning process replaces the CARO solution with an ozone solution (O3). The ozone solution is capable of removing hydrocarbons present on the surface of the wafer due to oxidation, is less expensive, is safer, and is an environmentally less dangerous solution. In addition, the ozone bath treatment does not require intermediate rinsing stages. Ultrasonic waves, also known as megasonic waves, can be used with the ozone solution to facilitate the removal of particles. Furthermore, the bonding between two wafers can initially be achieved via molecular adhesion, which is related to the hydrophilic properties of the bonding surfaces. However, after applying a treatment using a CARO solution or an ozone solution, followed by an RCA treatment, the hydrophilic properties of the bonding surface do not differ greatly from that present prior to using such a process.